Hierarchical differential image filters for skin analysis

ABSTRACT

There is provided a framework including systems and methods for analyzing skin parameters from images or videos showing skin. Using a series of Hierarchical Differential Image Filters (HDIF), it becomes possible to detect different skin features such as wrinkles, spots, and roughness. The hierarchical differential image filter computes two enhancements to an image showing skin at two different levels of enhancement, determines a differential image using the two enhancements and computes the skin analysis rating using the differential image. These skin ratings are comparably accurate to actual ratings by dermatologists.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 15/589,184 filedMay 8, 2017 (now U.S. Pat. No. 10,325,146 issued Jun. 18, 2019), andclaims the benefit of priority from U.S. Provisional Application No.62/333,258 filed May 8, 2016, the contents of each of which areincorporated herein by reference.

FIELD

The present application relates to image analysis, image filters,multi-scale image processing, video analysis, video filters and videoprocessing, and, in particular, skin analysis such as for dermatologicalpurposes.

BACKGROUND

The automatic evaluation and assessment of skin has been an area ofintense investigation for several decades [1-9]. There have beennumerous algorithms and methods to detect problem areas on the skin andto measure and monitor how these areas change over time [10]. In recentyears, a combination of factors, including the wide availability ofsmartphones with significant processing capability and a high definitioncamera, have increased the level of interest in automatic skinassessment [9].

Prior skin evaluation methods can generally be divided into threegroups. The first group utilizes image filters or transforms tohighlight specific concerns which can then be closely investigated onthe filtered image [2, 3, 4, 6]. These methods are fairly efficient,simple, and usually yield good results. A second group of methods enableusers to provide feedback on a particular area which is then closelyinvestigated (though region segmentation, color analysis, or othermethods) [12,13]. This provides a more focused and accurate evaluation,but does require user intervention which may not always be possible orideal. A third group of methods focus on machine learning for learningthe characteristics of different skin conditions which are then employedto classify different parts of the skin [1, 5, 7, 8, 11]. The lattermethod provides significant potential for automatic skin diagnosis, butdoes require extensive labelled skin images which are usually notavailable [7, 8, 11].

In this work, we focus on the first method, namely to apply ahierarchical filter to the skin image from which we extract quantitativecoefficients related to different set of general skin conditionsincluding texture/evenness, wrinkles, and spots. Our goal at this stageof our research is to obtain a high level understanding of the skinrather than focus on a particular skin anomaly. This work is indirectlyrelated (by similarity of subject matter) to our prior work on videofilters for skin evaluation [9], although the actual problem and methodspresented in this paper are entirely different than [9].

SUMMARY

There is provided A skin analysis unit comprising at least one processorconfigured to: analyze an image showing skin using a hierarchicaldifferential image filter to determine and provide a skin analysisrating, wherein the hierarchical differential image filter computes twoenhancements to the image at two different levels of enhancement,determines a differential image using the two enhancements and computesthe skin analysis rating using the differential image.

The two different levels of enhancement define a skin analysis levelconfigured to determine the skin analysis rating for a specific skinissue. The skin analysis unit may be configured to perform skin analysisusing different skin analysis levels, applying respective hierarchicaldifferential image filters. The different skin analysis levels maycomprise two or more of:

Level 1—facial texture (roughness and imperfections);

Level 2—dark spots and small wrinkles; and

Level 3—deep wrinkles and folds;

to determine skin analysis ratings for each specific skin issue.

The hierarchical differential image filter may computes the twoenhancements by applying a box blur function to the image at the twodifferent levels of enhancement.

An adjusted differential image at level i may be computed using adifferential image from each of level i and level i+1 to remove leakage.

The skin analysis unit may be configured to analyze a plurality ofrelated images from a video (e.g. successive images of a same area ofskin) using the hierarchical differential image filter to produce acandidate skin analysis rating for each of the plurality of imagesanalyzed. A final skin analysis rating may be determined by selecting amaximum candidate skin analysis rating.

Method and other aspects are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a differential image (right) showing thedifference between two different enhancement levels applied to theoriginal image (left).

FIG. 2 is an example of a hierarchical differential image set applied tothe original image (left), resulting in a first level image withroughness/texture (second from left), a second level image with darkspots (third from left), and a third level image with deep wrinkles(right).

FIG. 3 is a scatter plot illustrating the relationship between theindividual dermatologist ratings for skin roughness and the consensus(average) rating of all dermatologists' ratings for skin roughness.

FIG. 4 is a scatter plot illustrating the relationship between theindividual dermatologist ratings for dark spots and the consensus(average) rating of all dermatologists' ratings for dark spots.

FIG. 5 is a scatter plot illustrating the relationship between theindividual dermatologist ratings for deep wrinkles and the consensus(average) rating of all dermatologists' ratings for deep wrinkles.

FIG. 6 is a graph illustrating the number of images whose HDIF ratingestimates fall within a particular range of the dermatologist consensusratings.

FIG. 7 is a set of three images showing the effect of motion blur onHDIF skin ratings.

FIG. 8 is a block diagram of a computing unit configured as a skinanalysis unit in accordance with an embodiment.

FIG. 9 is a block diagram of a skin analysis module of a computing unitin accordance with an embodiment.

DETAILED DESCRIPTION

Hierarchical Differential Image Filters (HDIF)

Consider a skin image I(x,y) and an image enhancement functionΩ[I(x,y),u] with enhancement level u that can be applied to the image tocreate an enhanced image. The image enhancement could involve complexsteps such as removing specific problem areas of the skin, or couldinclude simpler smoothing or blurring effects. One computationallyefficient realization of the enhancement function would be a box blurwhich can be implemented efficiently using integral images. A box blur,(also known as a box linear filter) is a spatial domain linear filter inwhich each pixel in the resulting image has a value equal to the averagevalue of its neighbouring pixels in the input image. Other possibleenhancement functions include Gaussian blurring functions, local medianfunctions, histogram adjustments (where a function is applied to a localhistogram), edge-reduction approaches (where the edge of the regions areremoved as long as they are below a particular threshold), or acombination of the above. The enhancement level u corresponds to theintensity of the enhancement, with Ω[I(x,y),0]=I(x,y) (i.e. level 0enhancement does nothing to the image). In the case of using the boxblurring function, u would simply be the width of the blurring boxexpressed relative to the face size.

Now a differential image D_(u,v)(x,y), with v>u may be defined asfollows:D _(u,v)(x,y)=max(Ω[I(x,y),v)]−Ω[I(x,y),u],0)  (Eq. I)

Essentially, the differential image is the difference between twoenhancement levels with the idea that if the image is enhanced to removeproblem areas, then the difference between two enhancement levels wouldbe indicative of a particular type of skin problem. FIG. 1. is anexample of a differential image (right) showing the difference betweentwo different enhancement levels applied to the original image (left).The reason for the max operation is that most skin problem areas tend tobe darker than the actual skin color, and hence to find these darkerareas the maximum operation between successive enhancement levels wouldapply. For problem areas where the color is lighter than the skin tone,the max should be replaced by a −min (negative minimum) operation.

Observing the differential images, we can see that each image has aparticular focus on the size and type of skin problems, but there issometimes leakage from larger and more prominent problem areas. In orderto reduce this level leakage, we can define the following adjustedimages where the problem areas of the higher level i+1 are removed fromthe current level i:D* _(u) _(i) _(,v) _(i) (x,y)=max(D _(u) _(i) _(,v) _(i) (x,y)−D _(u)_(i+1) _(,v) _(i+1) (x,y),0)  (Eq. II)

Please note that for the highest level computed (analyzed), where thedifferential image for the next level (i.e. highest level+1) is notavailable, a default (or zero value) for the differential image for thenext highest level can be used. And finally, a level i skin ratingcoefficient that measures the total problems areas of the particularlevel can be computed or determined according to:

$\begin{matrix}{c_{i} = {\sum\limits_{x,{y \in S}}{D_{u_{i},\upsilon_{i}}^{*}\left( {x,y} \right)}}} & \left( {{Eg}.\mspace{11mu}{III}} \right)\end{matrix}$

where S is the set of skin pixels under evaluation. The skin analysisrating or coefficient may be normalized such as by scaling to apreferred range of values.

The S region may be defined based on the requirements of where the faceis to be analyzed. For example, if the goal is to evaluate skin on theforehead, then the forehead pixels would be included in the region S.Or, if the cheeks are under evaluation, then S would comprise the pixelson the cheeks. Automated operations to determine the set S given theregion of interest (e.g. forehead, cheeks, etc.) from a portrait imageare known to persons of ordinary skill. The set S could be defined fromuser input e.g. annotating the image or simply by capturing an image ofonly the region of interest (i.e. not a portrait).

In the case of a box blurring function, with the box width being definedas a ratio of the face width (i.e. u corresponds to a percentage of theface width), then for the following set of level differential pairs:

Level 1→{u₀,v₀}={0%, 2%}

Level 2→{u₁,v₁}={0%, 5%}

Level 3→{u₂,v₂}={7%, 12%}

differential images such as shown in FIG. 2 may be obtained where theleftmost images are the original images, and the respective Level 1, 2and 3 images progress rightward from the respective original images. Theoriginal image in the bottom row is an enlargement of a portion of theoriginal portrait image shown in the top row. As shown in FIG. 2,different levels are indicative of different facial features. Forexample, Level 1 image elements mostly correspond to the facial texture,roughness and imperfections. Unevenness of the skin is most evident inthis image. Level 2 image elements capture dark spots or small wrinkles(such as those adjacent to the lip). Level 3 image elements capture deepwrinkles and folds, such as the under eye wrinkle and nasolabial folds.Though the examples herein reference skin of a subject's face, otherskin surfaces may be similarly analyzed.

Dataset Generation

In order to evaluate our method's performance, we created a databaseconsisting of 49 portrait images with varying degrees of facialwrinkles, spots, discolorations, and texture. A panel of threedermatologists with expertise in skin treatment and evaluation wereassembled and asked to rate the conditions of each photo in the dataseton a 0-100% scale for each problem dimension (0%=no issues,100%=significant skin issues). Although the data collected involved moreextensive skin conditions, for the purposes of this paper, we will onlyevaluate the panel ratings for Deep Wrinkles, Skin Roughness, and DarkSpots.

Experimental Results

The first step in analyzing the data collected was to compare thedifferences in dermatologist ratings to the consensus (average) ratingfrom all dermatologists. FIGS. 3-5 are graphs (e.g. plots) illustratingthe rating differences between each dermatologist and the consensusratings. FIG. 3 is a scatter plot illustrating the relationship betweenthe individual dermatologist ratings for skin roughness and theconsensus (average) rating of all dermatologists' ratings for skinroughness. FIG. 4 is a scatter plot illustrating the relationshipbetween the individual dermatologist ratings for dark spots and theconsensus (average) rating of all dermatologists' ratings for darkspots. FIG. 5 is a scatter plot illustrating the relationship betweenthe individual dermatologist ratings for deep wrinkles and the consensus(average) rating of all dermatologists' ratings for deep wrinkles. Asshown in these graphs, there is a noticeable variation between theindividual dermatologist ratings and the consensus, indicating that evenan expert evaluation of the skin will have a moderate degree ofvariability.

In order to compare our proposed HDIF method with the ratings fromdermatologists, we ran the HDIF rating estimates on all 49 images. Wethen compared the average rating error from the HDIF ratings with thatof the consensus (average) rating from all dermatologists. Table I showsthe average rating errors for different skin concerns comparing theindividual dermatologists with the consensus (average) dermatologistrating, as well as HDIV skin rating estimates as compared to theconsensus rating:

TABLE I Average Rating Error For Individual Average Rating EvaluationDermatologist vs. Error For HDIF Dimension Consensus vs. Consensus Skin8.2% 11.3% Roughness Dark Spots 7.4% 17.6% Deep 6.5% 15.6% Wrinkles

As shown, the HDIF ratings have a higher error as compared to theindividual dermatologists. However, these errors are essentially within10% of the dermatologist ratings. Certain skin features such asroughness have a much closer HDIF estimate as compared to thedermatologist estimate. If we breakdown the ratings error among themoderate problem images (those with a consensus rating below 4) andintense (those with a consensus rating above 4), we see that moreintense problem areas usually correspond to higher errors for both HDIFand dermatologists. Table II shows a breakdown of dermatologist ratingerrors and HDIF rating errors for moderate and intense skin conditions:

TABLE II Average Rating Error For Individual Average RatingDermatologist vs. Error For HDIF Evaluation Consensus vs. ConsensusDimension Moderate Intense Moderate Intense Skin 7.4% 13.3%  10.0% 18.9%Roughness Dark Spots  7% 9.1%  16% 24.6% Deep 6.2% 7.6% 13.4% 22.3%Wrinkles

Another view of our data is to see what percentage of HDIF ratingestimates are within a particular range. FIG. 6 is a graph illustratingthe number of images whose HDIF rating estimates fall within aparticular range of the dermatologist consensus ratings. The best HDIFresults are obtained for skin roughness, followed by deep wrinkles anddark spots. For skin roughness over 88% of the dataset images had arating error less than 25%. For deep wrinkles, that number is 78% andfor dark spots it drops to 72%.

Based on our analysis, although dermatologists are clearly more capableat rating facial images than our HDIF approach, the difference betweenHDIF and the dermatologist consensus ratings is actually fairly small(having a max average difference of 10.2% for dark spot ratings). Thisindicates that our HDIF scores can be used as a possible metric for theassessment of skin.

Practical Considerations for Video

An intended goal for the skin evaluation framework disclosed herein isto apply the techniques to live video by performing scans on each frameand combining the results from successive frames. A first concern withsuch an implementation is the computational complexity of the processfor real-time performance on high definition video frames. Since the boxblur can be computed efficiently using integral images, and since allother steps are pixel additions and/or subtractions, the operations canbe composed of a series of addition/subtractions per pixel as well asmax calculations and memory transfer operations. As a result of theabove, the approach is well suited to real-time video implementation.

The second consideration with video is that of motion blur, which cancause blurring on the image thereby resulting in erroneous scores. FIG.7 is a set of three images of the same person with different levels ofmotion blur. The low blur image (left) has a HDIF Deep Wrinkle rating of8.7%. The moderate blur image (center) has a HDIF Deep Wrinkle rating of5.1%. The high blur image (right) has a HDIF Deep Wrinkle rating of4.0%. Similar drops were observed in the HDIF Roughness and Dark Spotratings.

As shown, the higher the level of motion blur the lower the skinratings. As a result, it is important to use video frames that have alow image blur. Since this is not always easily detectable, one simplesolution is to use the maximum ratings across a set of frames since themaximum rating usually corresponds to the least blurry frame.

Example Computing Unit Implementation

FIG. 8 is a block diagram showing a representative and simplifiedcomputing unit 800. Computing unit 800 comprises one or more processors802, camera 804, other input devices 806, communication units 808,display screen device 810, other output devices 812 and one or morestorage devices 814. Storage devices 814 store software (instructions toconfigure the one or more processors, communication units, etc.) as wellas data (e.g. source images or video, processed images or video, skinanalysis data and coefficients, etc.) Representative modules in storagedevices 814 include an image and video module (e.g. a camera module) fortaking and displaying images and video 816, a skin analysis module 818,other applications 820 and a communications module 822. Otherapplications may include a browser, email, instant messaging, SMS/MMS orother communication application, games, etc. A bus 824 couples thecomponents for communication. Display screen device 810 may be a touchscreen or otherwise configured for I/O operation. Other input devices806 may include a keyboard, one or more buttons, a microphone, biometricsensors, etc. Other output devices may include LED or other lights, aspeaker, an audio jack, bell, vibratory (haptic) devices, etc.Communication units may include radio and antenna components (e.g. forshort and/or long range wireless communication), USB or other interfacefor serial bus or other wired communication components, etc. Storagedevices 814 may be RAM, ROM, flash or other media types or other storagedevice components. Storage devices 814 typically store other softwarecomponents (not shown) such as operating system(s), etc., as will beappreciated by a person of ordinary skill. Processors are typicallymicroprocessors or CPUs. Other processing device configurations arepossible. While the instruction components are shown as software instorage devices, aspects may be configured as hardware.

Computing unit 800 may be a smartphone, lap top, work station, server,kiosk, or other computer. Though each of the components 802-824 ofcomputing unit 800 are shown “on board” some components such as anexternal/remote camera or external/remote display screen may be coupledto the computing unit 800 for communication. Preferably the camera iscapable of sufficient definition to show the skin issues, for examplewith 640×480 (e.g. VGA), or higher resolution. Though a single device isshown it is understood that the operations and methods discussed hereinmay be undertaken using more than one computing unit and all of same mayhave different configurations. For example, one computing unit maycapture an image (or video) and provide it to another computing unit foranalysis such as in a cloud computing configuration or otherclient-server arrangement (not shown).

FIG. 9 shows a block diagram of components 900 of the skin analysismodule 818 to process a source facial image 902. A flow of operationsmay be understood from same.

It is understood that the source image may be a video. Components 904and 906 are image processing components which perform image enhancementat respective levels U1 and U2 as discussed above. The image enhancementfunction may be a box blur function using one of the level pairs Level1, Level 2 or Level 3 described above. The enhanced images resultingfrom this respective processing are provided to a difference imagecomponent 908 to compute a difference image (for example using Eq. II asdiscussed above). The difference image is provided to a summationcomponent 910 to compute the image summation over skin regions ofinterest, for example, according to Eq. III. Though not shown a skinarea determining component may perform operations to determine the skinpixels from the image. The output thereof (e.g. a sum) may be normalized(e.g. defined over a preferred range of values) and a skin analysisrating or coefficient produced.

The skin analysis module 818 may be configured to determine a skinanalysis coefficient using each or only some of the Level 1, 2 and 3pairs to determine 1) the facial texture, roughness and imperfections(unevenness of the skin), 2) dark spots or small wrinkles, and/or 3)deep wrinkles and folds respectively. The analysis may be selective(e.g. to allow a user to choose the level to be used or the skin issueto be analyzed). Though not shown the coefficient may be provided to auser of computing unit 800 such as via display screen device 810 orcommunicated to another computing unit for presentation or stored to adata store (not shown) for later retrieval and/or other use.

Computing unit 800 configured to determine the skin analysis coefficientmay be considered to be a skin analysis unit. The skin analysis unit isthus one or more processors configured to analyze an image showing skinusing a hierarchical differential image filter to determine a skinanalysis rating. The hierarchical differential image filter computes twoenhancements to the image of the skin at respective levels ofenhancement, determines a differential image from the two enhancementsand computes a skin analysis rating from the differential image. Theimage may a still image or an image from a video.

In a video context, a plurality of related images from a video (e.g.successive images of the same area of skin) may be analysed to produce aplurality of candidate skin analysis ratings, one for each of theplurality of images analyzed. A final skin analysis rating may bedetermined by selecting a maximum rating from the plurality of candidateskin analysis ratings so as to account for image blur in the relatedimages.

The image may be analyzed at different enhancement levels (e.g. Level 1,Level 2, Level 3) to provide skin analysis ratings for respective skinissues. The analysis may be selective to analyze one or more selectedlevels or skin issues.

One or more method aspects will be apparent from the foregoingdescription. A computer storage product may be defined to configure acomputing unit having one or more processors to be a skin analysis unit.The storage product may be a storage device storing instructions in anon-transient manner which instructions when executed configure the oneor more processors to perform as described herein.

CONCLUSION

A framework including systems and methods for analyzing skin parametersfrom images or videos is disclosed. Using a series of HierarchicalDifferential Image Filters, different skin features such as wrinkles,spots, and roughness are detectable and skin ratings may be computed. Itwill be apparent that modifications may be made by a person of ordinaryskill to the teachings herein yet remain within the scope. For example,the image may not be a portrait per se of the subject. The image may bea portion of a face or other area of the subject's body. The image mayinclude other surfaces other than skin, such as clothing, accessories,background, glasses, etc. which other surfaces may be removed from therating analysis such as by defining the set S appropriately.

REFERENCES

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The invention claimed is:
 1. A skin analysis unit comprising at leastone processor configured to: analyze a plurality of related images froma video showing skin using a hierarchical differential image filter todetermine a plurality of candidate skin analysis ratings from each imageof the plurality of related images, wherein the hierarchicaldifferential image filter computes two enhancements to each image at twodifferent levels of enhancement, determines a differential image usingthe two enhancements and computes one of the candidate skin analysisratings from the differential image; and determine and provide a finalskin analysis rating from the plurality of candidate skin analysisratings.
 2. The skin analysis unit of claim 1 wherein the final skinanalysis rating is determined by selecting a maximum rating from theplurality of candidate skin analysis ratings.
 3. The skin analysis unitof claim 1 wherein the plurality of related images are selected fromsuccessive images of a same area of skin from the video.
 4. The skinanalysis unit of claim 1 wherein the two different levels of enhancementdefine a skin analysis level configured to determine each of thecandidate skin analysis ratings for a specific skin issue.
 5. The skinanalysis unit of claim 4 configured to perform skin analysis usingdifferent skin analysis levels, applying respective hierarchicaldifferential image filters wherein the different skin analysis levelscomprise two or more of: Level 1—facial texture comprising roughness andimperfections; Level 2—dark spots and small wrinkles; and Level 3—deepwrinkles and folds; to determine respective skin analysis ratings foreach specific skin issue.
 6. The skin analysis unit of claim 1 whereinthe hierarchical differential image filter computes the two enhancementsby applying a box blur function to each image of the plurality ofrelated images at the two different levels of enhancement.
 7. The skinanalysis unit of claim 1 wherein each image of the plurality of imagesis respectively defined by I(x,y) and the hierarchical differentialimage filter applies an image enhancement function Ω[I(x,y),u] withenhancement level u corresponding to the intensity of the enhancement,with Ω[I(x,y),0]=I(x,y).
 8. The skin analysis unit of claim 7 whereineach respective differential image is generally defined by D_(u,v)(x,y),with v>u such that:D _(u,v)(x,y)=max(Ω[I(x,y),v)]−Ω[I(x,y),u],0)  (Eq. I) wherein a maxoperation is performed for skin problem areas that are darker thanactual skin tone and a −min (negative minimum) operation is performedfor skin problem areas that are lighter than actual skin tone.
 9. Theskin analysis unit of claim 8 configured to analyse each image of theplurality of related images at different skin analysis levels i and i+1using respective hierarchical differential image filters and wherein thedifferential image for level i is determined from an adjusted imagewhere skin problem areas of the higher level i+1 are removed from thecurrent level i.
 10. The skin analysis unit of claim 1 wherein: theimage enhancement function is a box blurring function having a box widthdetermining a level of enhancement (i) applied to each image of theplurality of related images; and for each level i, the hierarchicaldifferential image filter applies box widths u_(i) and v_(i) where eachrespective box width is defined as a respective ratio of a face width ofa face in each image of the plurality of related images.
 11. The skinanalysis unit of claim 10 configured to perform skin analysis using atleast one of a plurality of different skin analysis levels, applyingrespective hierarchical differential image filters wherein the differentskin analysis levels are selected from: Level 1→{u₀,v₀}={0%, 2%}; Level2→{u₁,v₁}={0%, 5%}; and Level 3→{u₂,v₂}={7%, 12%}.
 12. The skin analysisunit of claim 1 configured to normalize the final skin analysis rating.13. The skin analysis unit of claim 1 comprising either an on-boardcamera or a remote camera coupled to the unit and wherein the unit isconfigured to capture the video using the on-board camera or remotecamera.
 14. A method of skin analysis comprising: analyzing a pluralityof related images from a video showing skin using a hierarchicaldifferential image filter to determine a plurality of candidate skinanalysis ratings from each image of the plurality of related images,wherein the hierarchical differential image filter computes twoenhancements to each image at two different levels of enhancement,determines a differential image using the two enhancements and computesone of the candidate skin analysis ratings from the differential image;and determine and provide a final skin analysis rating from theplurality of candidate skin analysis ratings.
 15. The method of claim 14wherein determining the final skin analysis rating comprises selecting amaximum rating from the plurality of candidate skin analysis ratings.16. The method of claim 14 comprising selecting the plurality of relatedimages from successive images of a same area of skin from the video. 17.The method of claim 14 wherein the two different levels of enhancementdefine a skin analysis level configured to determine each of thecandidate skin analysis ratings for a specific skin issue.
 18. Themethod of claim 17 comprising performing skin analysis using differentskin analysis levels, applying respective hierarchical differentialimage filters wherein the different skin analysis levels comprise two ormore of: Level 1—facial texture comprising roughness and imperfections;Level 2—dark spots and small wrinkles; and Level 3—deep wrinkles andfolds; to determine respective skin analysis ratings for each specificskin issue.
 19. The method of claim 14 comprising using the hierarchicaldifferential image filter to compute the two enhancements by applying abox blur function to each image of the plurality of images at the twodifferent levels of enhancement.
 20. The method of claim 14 wherein eachof the plurality of images of the skin is respectively defined by I(x,y)and wherein using the hierarchical differential image filter applies animage enhancement function Ω[I(x,y),u] with enhancement level ucorresponding to the intensity of the enhancement, withΩ[I(x,y),0]=I(x,y).
 21. The method of claim 20 wherein each respectivedifferential image is generally defined by D_(u,v)(x,y), with v>u suchthat:D _(u,v)(x,y)=max(Ω[I(x,y),v)]−Ω[I(x,y),u],0)  (Eq. I) wherein a maxoperation is performed for skin problem areas that are darker thanactual skin tone and a −min (negative minimum) operation is performedfor skin problem areas that are lighter than actual skin tone.
 22. Themethod of claim 21 comprising analysing each image of the plurality ofrelated images at different skin analysis levels i and i+1 usingrespective hierarchical differential image filters and wherein thedifferential image for level i is determined from an adjusted imagewhere skin problem areas of the higher level i+1 are removed from thecurrent level i.
 23. The method of claim 14 wherein: the imageenhancement function is a box blurring function having a box widthdetermining a level of enhancement (i) applied to each image of theplurality of related images; and for each level i, the hierarchicaldifferential image filter applies box widths u_(i) and v_(i) where eachrespective box width is defined as a respective ratio of a face width ofa face in each image of the plurality of related images.
 24. The methodof claim 23 comprising performing skin analysis using at least one of aplurality of different skin analysis levels, applying respectivehierarchical differential image filters wherein the different skinanalysis levels are selected from: Level 1→{u₀,v₀}={0%, 2%}; Level2→{u₁,v₁}={0%, 5%}; and Level 3→{u₂,v₂}={7%, 12%}.
 25. The method ofclaim 14 comprising normalizing the final skin analysis rating.
 26. Themethod of claim 14 comprising capture the video using a camera on-boardor coupled to a skin analysis unit configured to perform the method. 27.A computer program product comprising a storage device storinginstructions in a non-transient manner to configure one or moreprocessors when executed to provide a skin analysis unit, the one ormore processors configured to: analyze a plurality of related imagesfrom a video showing skin using a hierarchical differential image filterto determine a plurality of candidate skin analysis ratings from eachimage of the plurality of related images, wherein the hierarchicaldifferential image filter computes two enhancements to each image at twodifferent levels of enhancement, determines a differential image usingthe two enhancements and computes one of the candidate skin analysisratings from the differential image; and determine and provide a finalskin analysis rating from the plurality of candidate skin analysisratings.